1. Field of the Invention
The present invention relates to an image processing apparatus and image processing method for generating a tone-converted image from a color image.
2. Description of the Related Art
When a user photographs a certain scene with a digital camera outdoors, the luminance range of the target scene is sometimes wider than a photographable luminance range. In this case, tone information of an object outside the photographable luminance range cannot be recorded, so a highlight detail loss or shadow detail loss occurs. For example, when a person is photographed outdoors on a fine day while the exposure is adjusted to him, a highlight detail loss may occur in the background including the sky and clouds, or a shadow detail loss may occur in the shade of trees.
On the other hand, human vision has a characteristic called “local adaptation” that switches the adaptation state according to the brightness of a region one views to perceive the brightness and color. With this characteristic, one can perceive tones in both bright and dark places. For this reason, an impression upon directly viewing a photographed image suffering highlight detail loss or shadow detail loss sometimes differs from that upon viewing an actual scene. This has been a complaint of some digital camera users.
One technique for solving this problem is known as a HDR (High Dynamic Range) technique. The HDR technique roughly includes an HDR capture technique and HDR reproduction technique. The HDR capture technique is used to expand the photographable dynamic range and record tone information of a luminance range suffering a highlight detail loss or shadow detail loss. An example of this technique is a method of compositing images photographed at a plurality of exposure values. An image acquired by the HDR capture technique will be called an HDR image. The HDR reproduction technique is a dynamic range compression technique for reproducing an HDR image having a wide dynamic range by a display/output device having a narrow dynamic range. An example of this technique is a method of compressing the low-frequency components of an HDR image. The HDR technique can reduce highlight detail losses and shadow detail losses by the capture technique for expanding the dynamic range and a corresponding reproduction technique.
A variety of methods have been proposed as dynamic range compression techniques. For example, “iCAM06” proposed by J. Kuang et al. is a dynamic range compression technique based on an idea of reproducing, by a display/output device, an impression when one views a scene (see, e.g., Kuang, J., Johnson, G. M., Fairchild, M. D., “iCAM06: A refined image appearance model for HDR image rendering”, Journal of Visual Communication, 2007). In “iCAM06”, the appearance of the brightness and color perceived by a viewer in a photographing scene is first simulated using an HDR image. Then, the brightness and color are converted into those reproducible by the output device. Finally, the reproducible brightness and color are converted into signal values for the display/output device. By compressing the tone of an original image using this technique, a low dynamic range device (LDR device) can reproduce a satisfactory tone.
With a tone conversion technique such as “iCAM06”, the LDR device can achieve satisfactory tone reproduction. However, the following problem arises when an HDR device displays an image (LDR image) tone-converted for LDR. More specifically, an LDR image does not hold reconstruction information for an HDR image. When displaying an LDR image on an HDR device, the signal value of the LDR image is directly input to the HDR device. Hence, the LDR image displayed on the HDR device cannot reproduce the original tone.